1. Field of the Invention
The present invention relates generally to a flashlight and more particularly to an LED flashlight which allows continuously variable beam diameters from spot to wide angle.
2. Description of the Related Art
A typical prior art flashlight employs a conical-shaped reflector oriented behind an incandescent light bulb to provide a forward focused pattern of light around a target object. The uneven brightness and on and off-axis bright spots are caused by imperfections in the surface of the reflector. The light pattern provided by such an arrangement is usually uneven in brightness with on and off-axis bright spots. Inconsistent light patterns are lower in overall light intensity which hampers the identification of the target object to be viewed.
Today flashlights use mostly light-emitting diodes as light sources. For example, U.S. Pat. No. 7,461,945 discloses a focus-adjustable LED flashlight comprising a tubular housing; an LED light-emitting unit coupled to the tubular housing; a sleeve unit coupled movably to the tubular housing; and a positive lens mounted in the sleeve unit and spaced apart from and aligned with the LED light-emitting unit along an axis of the tubular housing. The sleeve unit is movable relative to the tubular housing and the LED light-emitting unit along the axis of the tubular housing so as to adjust a distance between the positive lens and the LED light-emitting unit. Specifically, the positive lens is preferably a plano-convex lens, as mentioned in that patent. It is because the plano-convex lens shows minimum spherical aberration when oriented with its plane surface facing the light source, when compared with a usual symmetric convex singlet or a reversed plano-convex lens. Furthermore, since a plano-convex lens costs much less to manufacture than a biconvex lens, the plano-convex lens is quite popular.
However, in practice, it is difficult to make a thick plano-convex lens, with a desired short focal distance, by injection-molding from a polymer material because the convex side of the plano-convex lens would be highly curved and thus the lens become relatively thick across its middle and thin at its upper and lower edges. That is to say, when molding such thick lens, problematic short shots occur because the polymer melt cannot fill the entire cavity. Thus, nowadays flashlights use mostly plano-convex lenses formed from a glass material.